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Talking hyperthetically, suppose I had a non-legitimate version of Windows XP, but I have now obtained a legitimate key (it belonged to an old computer that has now been trashed). How can I make the version legitimate without having to reinstall?

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Whether a non-legit XP can be activated depends on what version it is. If it's a "Corporate" Professional version, then it never needed to be activated anyway, and no matter how much you try, you won't be able to deactivate it. But otherwise, you can follow the method suggested. – Mussnoon Nov 22 '10 at 9:06
up vote 3 down vote accepted

For the solution given by whitequark to work, the two XP versions must be of the same kind, since one cannot activate an non-OEM version with OEM serial.

If the versions don't match, the solution will be to reinstall XP using the boot CD of the thrashed computer (if you still have it), or search elsewhere. It is not illegal to download via torrent a copy of a software that you own legally.

However, if the thrashed computer had an OEM version, then the license is "hardwired" to the computer and cannot be used on another, so the activation may not work.

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If I were to torrent a version, is there any way I can know that it doesn't have a virus or been otherwise modified? – Casebash Nov 22 '10 at 9:21
No way, except by looking for a suitable torrent on community sites where there are enough comments vouching for it. And of course running a few full antivirus scans as first thing. – harrymc Nov 22 '10 at 10:00
@Casebash: You can go to and there you should be able to see SHA-1 for all media that was released by Microsoft. Once you download ISO from torrents just use some SHA-1 calculator on your side (e.g.Summae at and compare sum that you got with official one. If they match, you have unmodified CD/DVD. – Josip Medved Nov 22 '10 at 12:31

This article describes how to de-activate your system. Then you can just launch the standard activation process again.

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